As one can ascertain, there are many medical procedures where the knowledge of the position of a catheter is desirable. One such example is in positioning a balloon angioplasty catheter. In such a system one portion of the catheter utilized expands upon inflation thereby opening up a narrow section of an artery which artery contains plaque. The objective is positioning the catheter so that the balloon portion is where the plaque is. According to present techniques of providing such positioning, one employs X-ray fluoroscopy. In X-ray fluoroscopy a contrast agent which is visible under X-ray radiation is sent through the catheter and its tip and is made visible by the use of X-ray fluoroscopy. Such methods of positioning a catheter and other interventional systems employ X-ray techniques to locate the position of the catheter within the body cavity. As one can understand, the drawbacks of X-ray visualization include the potential harmful effects of the radiation to the patient and attending physicians. There is potential for additional harmful effects due to the contrast agent. Another major concern is the cost of the X-ray apparatus and personnel.
Certain other prior art techniques utilize highly radiopaque tantalum or other metal markers which identify the area of effective dilatation. These tantalum markers are firmly anchored on the catheter shaft to enable one to detect the position of the catheter by means of X-ray.
The prior art was cognizant of the harmful effects of X-ray radiation and hence, prior art approaches attempted to utilize ultrasound or other imaging techniques which were well suited for soft tissue analysis and which techniques presented no X-ray hazard. In this regard, reference is made to U.S. Pat. No. 4,249,539 entitled ULTRASOUND NEEDLE TIP LOCALIZATION SYSTEM issued on Feb. 10, 1981 to D. H. R. Vilkomerson, et al. This patent describes a system which detects the tip of an aspiration needle used in an aspiration procedure and shows the tip in an ultrasound image by means of a transducer removably positioned at the tip. The patent describes utilizing the transducer as a transponder and thereby sending a signal back through the body to the transmitter when a signal is detected. In the patent, the aspiration needle removably carries a small, omnidirectional ultrasound transducer which is electrically connected through the needle to transponder electronics. Incident pulses from the imaging transducer to the hemispherical transducer at the needle tip are sensed at the latter and the aspiration needle position is inserted into the image either by generation of a return signal emitted from the needle point, "direct transponding" or by sending an appropriately delayed signal directly to the transmitting system via a wire rather than through the body "indirect transponding".
Reference is also made to U.S. Pat. No. 4,706,681 entitled CARDIAC ULTRASONICALLY MARKED LEADS AND METHOD FOR USED SAME issued on Nov. 17, 1987 to B. Breyer, et al. This patent describes ultrasonically marked leads produced by mounting one or more piezoelectric marker transducers into the leads and connecting the transducers by electrical conductors to appropriate electric circuits which upon reception of the scanner ultrasonic signals by the marker transducers generate appropriate electric signals. These signals localize the marker transducers in an ultrasonic echographic image, thereby permitting guiding of pacing leads and detection of malfunctions.
As one can see from this patent, the marker transducers are cylindrical or tubiform in shape. These transducers are flat in regard to their surface configuration and in all cases the cylinders are longer than the diameter. In some cases the cylinders are twice as long as the diameter to conform to the definition of tubiform. Such tubular transducers are sensitive only to beams that are close to perpendicular to their long axis. As will be further explained, the angle over which such a transducer has significant sensitivity is extremely small.
The tubiform transducers shown in U.S. Pat. No. 4,706,681 are incapable of responding to energy which impinges from angles other than energy directed perpendicular to the axis of the transducer. As will be described, such transducers can not be used to reliably locate the position of a catheter. The hemispherical transducer of 539 cannot be mounted on the tubular portion of a catheter.
The present invention, a curved annular transducer, circumvents the prior art problems: the transducer to be described is both sensitive over a broad range of angles of incident acoustic beams, and mountable on a tube in any required location. In this manner, it enables an ultrasonic imaging system to show the position of the transducer on a tubular device during normal scanning modes.